You make the Call !!
 

I have not seen this in over 30 years.

Team A has the ball 1st and 10 on B's 25 yard line going in. Team B's player has his helmet come off at the 18 yard line and continues to play the whole down all the way to the endzone. Team A looses the ball on the 3 yard line and goes into the endzone where Team B recovers.

Where would you enforce this IP at??

You have a foul by the defense during a run which ended when the ball came loose. So the enforcement is half the distance from B's 3 yard line, and the change of possession is nullified.

To be a foul, though, the player whose helmet comes off has to materially affect subsequent play, which is a condition of "participation". Theoretically any time during that interval where that player affects play could be another "instance" of IP, but it's hard to imagine calling it at any other time would benefit the non-offending team more. Play could be affected by such things as other players pulling up instead of hitting him, or there could be more overt actions, but I could imagine situations in which that player didn't affect play at all, so you'd have to see it.

Disagree. He does not have to materially affect the play to participate. He must "cease involvement with the play." NFHS 9.6.4 Situation F

Fine, but what does "involvement" mean? 1-5-1 says, "Each player shall participate while wearing the following pieces of properly fitted equipment,"

The foul in question is a violation of the "illegal participation" provisions, under 9-6-4(g): "For a player whose helmet comes completely off during a down to continue to participate beyond the immediate action in which the player is engaged."

2-30 defines "participation" as "any act or action by a player or nonplayer that has an influence on play."

Clearly "any action...that has an influence on play" can't be understood literally, or it would include people's sending signals in from the bench. That's why I use the phrase "materially affect"; signals are not material.

I agree with this. If he pursues the player with the ball, it's a foul. It's not hard to discern this.

I'd agree that it'd be an easy call if one took "continues to play the whole down all the way to the endzone" to mean that he ran into the end zone. Since the play ended with the ball in the end zone, I just took it as meaning, "until the ball reached the end zone", during which time he might've been picking his nose or whatever.